The panic over the Heartbleed bug is proving to be a convenient distraction for hackers using standard techniques in a fresh wave of attacks targeting at least 18 U.S. universities, according to a computer security researcher.

A hospital connected with a university is on the verge of resetting passwords for 17,000 people after a phishing email circulated that asked people for their account credentials.

"Somebody had access to a student account that had an entire list of employees," said Alex Holden, CTO of Hold Security, which specializes in tracking data breaches in the cybercriminal underground.

It's unknown how much data has been pilfered from the schools or the extent to which the schools are aware of the attacks. Up to 250,000 people could be affected, Holden said.

"Hopefully, it's not payroll [data]," he said.

The hospital considering a mass password reset is a client of Hold Security. The other universities have not been notified.

"We have had very bad luck with several universities as far as reaching out about breaches," Holden said. "The problem is finding within a very large, decentralized infrastructure the right person to talk to and impress on them that something is going on."

The attackers have been using SQL injection attacks, a standard attack method that tries to see if backend databases will surrender data through different types of requests. In other cases, it appears backdoors, or malicious software programs that can steal data, have somehow been installed.

Verizon attributed 35 percent of 1,300 data breaches it examined that occurred in 2013 to be attacks on web applications in its 2014 Data Breach Investigations Report, which was released on Wednesday.

Holden, who monitors password-protected forums to gain clues as to the interests of hackers, said they've been strangely quiet about Heartbleed, the flaw in the OpenSSL cryptographic library that can be used to compromise encrypted content exchange between clients and servers.

He likens the silence to keeping a good fishing spot secret: no hacker wants to publicly point out a soft target to the masses.

Instead, he's seen an upsurge in sales of lists of domains that can be attacked. Finding domains and subdomains of large companies or universities can involve a lot of manual research and guesswork.

Between US$20 to $100 can buy a list a of millions of domains, which can be attacked. Holden suspects the lists may come from insiders at registrars. The practice of aggregating domains isn't in itself illegal, he said.

Universities run very large networks. While compromising a university network may not be as profitable as other targets, hackers would get access to large numbers of young users, 10,000 upwards to 50,000.

"They may not have a lot of money in their accounts but when exploited in bulk they may be as profitable as a number of people further in their careers," Holden said.